Add support for Mixxx's JS controller scripts

This is probably a bit more involved than #1 since it involves running the script with the same API as Mixxx (e.g. providing an `engine` that interfaces with the DJ application).

Since most (all?) invocations of the script are quick one-shot calls to functions mapped in the XML script, IIUC, we might be able to 'collect' a list of actions to take that we can return to the consumer of this library, instead of requiring them to provide callbacks/an interface to us.

The type declarations here might be interesting:
- https://github.com/mixxxdj/mixxx/pull/4759

In-script APIs to implement:

- [ ] `engine` (partially implemented)
- [ ] `getValue`
- [ ] `setValue` (partially implemented)
- [ ] `setParameter` (partially implemented)
- [ ] `script` (partially implemented)

To be investigate:

- Could we use Web Workers or some other means of isolation to properly sandbox the scripts while still providing them with context to interact with the app?